Third generation wireless communication systems include downlink (i.e., communication link from a base station to a mobile-station) performance improvement techniques. One technique for improving downlink performance is to use, at the base station, a transmit diversity scheme (also referred to herein as diversity gain).
Transmit diversity can be used to improve fading distribution in the received signal, and is typically achieved using an antenna array configuration having M antenna elements spaced far apart and transmitting identical signals. By spacing the antenna elements far apart, fading is distributed since each of the signals will travel a different path from its transmitting antenna element to the mobile-station and experience a different distortion or fading process. Thus, the mobile-station receives identical signals affected by different fading processes or distortions. Since each signal should experience a different distortion or fading process, it becomes less probable that all the signals will undergo deep fades. Thus, fading distribution is improved.
When the received signals are properly combined by the mobile-station, the result is a signal with an improved bit error rate due to improved fading distribution although average signal-to-noise ratio remains approximately the same as if transmit diversity was not employed. In order to combine the received signals, the mobile-station needs to be capable of separating the individual received signals from each other. Separating the individual received signals is difficult if the signals were transmitted using a same frequency. Different frequencies may be used to transmit the individual signals such that separation of the received signals is easier. However, such method consumes additional bandwidth, which is undesirable. A same frequency may be used to transmit the signals in code division multiple access (CDMA) systems employing delay diversity techniques, which would allow signals to be separated through long spreading codes. Although additional bandwidth is not consumed, this technique causes mutual interference due to multipaths being intentionally created by the delay diversity techniques.
One technique for avoiding the mutual interference problem is orthogonal transmit diversity, which applies only to coded systems. Orthogonal transmit diversity involves transmitting alternate data bits modulated with different Walsh codes from different antenna elements. Diversity is gained in the decoding process when a convolutional code is employed with a Viterbi decoder, but not on the data bits itself since the antenna elements are transmitting only alternate data bits (and not all the data bits or the entire signal). In systems with weak convolutional or other error correction codes (for example, due to puncturing), the performance gained by orthogonal transmit diversity techniques may degrade.
The weakness of orthogonal transmit diversity may be overcome using a technique referred to herein as space time spreading (STS). STS involves transmitting all data bits (but not necessarily the same representations of the data bits) on two antenna elements using different Walsh codes, thus diversity is achieved on the data bits. No coding is required to achieved diversity (although coding may still be used), thus diversity will not degrade if coding is weak.
FIG. 1 depicts a wireless communication system 10 employing STS. Wireless communication system 10 comprises at least one base station 12 having two antenna elements 14-1 and 14-2, wherein antenna elements 14-1 and 14-2 are spaced far apart for achieving transmit diversity. Base station 12 receives a signal S for transmitting to mobile-station 16. Signal S is alternately divided into signals se and so, wherein signal se comprises even data bits and signal so comprises odd data bits. Signals se and so are processed to produce signals S14-1 and S14-2. Specifically, se is multiplied with Walsh code w1 to produce signal sew1; a conjugate of signal so is multiplied with Walsh code w2 to produce signal so*w2; signal so is multiplied with Walsh code w1 to produce sow1; and a conjugate of signal se is multiplied with Walsh code w2 to produce se*w2. Signal sew1 is added to signal so*w2 to produce signal S14-1 (i.e., S14-1=sew1+so*w2) and signal se*w2 is subtracted from signal sow1 to produce signal S14-2 (i.e., S14-2=sow1xe2x88x92se*w2) Signals S14-1 and S14-2 are transmitted over antenna elements 14-1 and 14-2, respectively.
Mobile-station 16 receives signal R comprising xcex31(S14-2)+xcex32(S14-2), wherein xcex31 and xcex32 are distortion factor coefficients associated with the transmission of signals S14-1 and S14-2 from antenna elements 14-1 and 14-2 to mobile-station 16, respectively. Distortion factor coefficients xcex31 and xcex32 can be estimated using pilot signals, as is well-known in the art. Mobile-station 16 decodes signal R with Walsh codes w1 and w2 to respectively produce outputs:
W1=xcex31se+xcex32soxe2x80x83xe2x80x83equation 1
W2=xcex31so*xe2x88x92xcex32se*xe2x80x83xe2x80x83equation 1a
Using the following equations, estimates of signals se and so, i.e., ŝe and ŝo, may be obtained:
ŝe=xcex31*W1xe2x88x92xcex32W2*=se(|xcex31|2+|xcex32|2)+noisexe2x80x83xe2x80x83equation 2
ŝo=xcex32*W1+xcex31W2*=so(|xcex31|2+|xcex32|2)+noisexe2x80x2xe2x80x83xe2x80x83equation 2a
STS, however, does not scale naturally to more than two antenna elements to enhance diversity gain without having to reduce data rate. Accordingly, there exists a need to enhance diversity gain without reducing data rate.
The present invention is a method and apparatus for enhancing diversity gain without reducing data rate by increasing the number of antenna elements for purposes of improving signal-to-noise ratio at a receiver. In one embodiment, the present invention is employed using an antenna array having a first antenna group with at least two antenna elements and a second antenna group with at least one antenna element. The first and second antenna groups are spaced approximately ten carrier wavelengths or more apart from each other, and the antenna elements belonging to the first antenna group are spaced approximately a half carrier wavelength or less apart from each other.
A signal is processed for transmission over the antenna array by first generating a plurality of data streams from the signal. A first plurality of representative data streams is derived from the plurality of data streams, and a second plurality of representative data streams is derived from the plurality of data streams. Each of the first plurality of representative data streams is phase-shifted and encoded using different orthogonal codes, and each of the second plurality of representative data streams is encoded using different orthogonal codes, wherein a different orthogonal code is used to encode representative data streams of the first and second plurality of representative data streams generated from a same data stream of the plurality of data streams, and the first and second plurality of representative data streams are representatives of the plurality of data streams that allow for the plurality of data streams to be recovered at a receiver after encoding and transmission. The encoded and phase shifted first plurality of representative data streams being transmitted over the first antenna group, and the encoded second plurality of representative data streams being transmitted over the second antenna group.
In one embodiment of the present invention, a pilot signal is transmitted along with the encoded and phase shifted first plurality of representative data streams over each antenna element belonging to the first antenna group, and a pilot signal is transmitted along with the encoded plurality of representative data streams being transmitted over each antenna element belonging to the first antenna group. The pilot signal being transmitted over each antenna element in the first and second antenna groups may be identical or different (in terms of orthogonal codes being applied to each pilot signal or sequence of bits comprising each pilot signal).